Okay, decision made.
I've ordered this Garmin Nuvi 1410 refurbed
Weatherproof case, RAM & SW mo-tech compatible for 5" sat navs
12v waterproof socket with a faceplate and a fuse
I've also got an appointment to get one of these fitted tomorrow, they are only round the corner so he's going to drill the top yoke bolt and fit it while I wait.
Total cost - £140
That's cheaper than the bike specific ones are going second hand; it's a bit of a joke how expensive they are.